The present invention relates to a method and an apparatus for transporting preprinted web-shaped recording media, particularly of continuous form paper, in an electrographic printing device, in particular. Paper, film material, labels or other materials can be used as the web-shaped recording medium.
The greatest variety of paper grades are employed when printing paper. What is referred to as margin-perforated paper is mainly employed in the electrographic high-performance printer field with printing outputs of more than 40 pages per minute. This paper has lateral holes at its longitudinal edges for transport and for monitoring the position of the paper. It is thereby driven by sprocket tractors that engage into the lateral transport holes. This paper often also has transverse perforations along which the individual pages are separated from one another.
The margin perforation is particularly employed when processing preprinted paper. Given this paper, the information subsequently applied in the electrographic printer, for example data that are printed on a pre-printed form, should come to lie as exactly as possible at predetermined locations of the pre-print. The form can have been preprinted by a printing device of another construction, for instance by an offset printer, or by an identical electrographic printer. For positionally exact printing, the position of the paper web in the conveying direction must be exactly adjusted to or, synchronized with the drive thereof or, the movement of the photoconductor drum.
For exact positioning of such paper, the first page of the paper web is placed exactly at a specific position with respect to the sprocket tractors. A page start mark of the pre-print or, of, the transverse perforation thereby exactly prescribes the beginning of the page. All further pages are automatically exactly positioned due to the constrained guidance by the tractor sprocket when the first page was properly inserted.
The feed of the perforated paper usually occurs in a specific grid corresponding to the hole spacings, for example in a 1/2+L inch grid or in a 1/6+L inch grid. The paper web is then not moved continuously but step-by-step by a multiple of the grid spacing.
There is frequently also the demand in the high-performance printing field to be able to employ roll paper that does not comprise such margin perforations in printers for continuous-form paper. Both economic as well as ecological considerations contribute to this demand. When printing margin perforated paper, namely, a processing step wherein the margin strips are removed from the printed page is necessary, whereby the waste that thereby arises must be disposed of.
For example, WO 95/19929 A1 discloses a printer that is suitable for processing roll paper without margin perforation. A first seating edge, which prescribes the lateral position of the paper, as well as stabilization rollers, an under-pressure brake and a roller arrangement with a loop-drawing means are provided in this printer for the exact transport of the paper.
Even though continuous form papers both with and without margin perforation can be fundamentally processed with such a device, problems arise when printing forms. When one wishes to process pre-printed paper with such a printer, then no direct allocation of the pre-printed area to the information to be subsequently printed is possible. As a result thereof, the information to be subsequently printed cannot be fitted into the pre-print in positionally correct fashion.
Causes of mispositioning are, for example, fluctuations in the paper length that derive from different ambient temperatures or different degrees of moisture of the paper web. Such fluctuations can amount to up to a few millimeters per page. Deviations in the print image on this order of magnitude are not acceptable when printing forms.
Added thereto given tractor-less friction drives is the problem that the transport precision in the feed direction cannot always be adhered to. For example, slippage between the drive drum and the paper web or manufacturing tolerances of the drive mechanism can contribute thereto.
DE 19 37 699 A likewise discloses a friction drive for data printers. A sensor that recognizes a pre-printed mark at the edge of the form is provided given this drive. A reallocation of the line height to the printing location is undertaken with the sensor result with the respective start of the form. What is disadvantageous about this drive is that a mark adapted to the sensor must be pre-printed at a specific position of the form so that the control function can be implemented.
CH 608 904 A5 teaches an apparatus with which it is possible to write a running band of forms with individual data. For this purpose, a sensor is provided, which detects a particular printed feature in the forms and then triggers the print process within a particular field in the forms. The disadvantage of this apparatus is that, on the one hand, the feature on the forms must have a particular design, for instance a rectangular shape, in order to be reliably detected by the sensor. On the other hand, the measurement accuracy of the sensor strongly depends on which recording medium is used. In particular, the contrast of a mark relative to the background (e.g. to a paper surface) can differ significantly from one recording medium to the next (e.g. given different types of paper).
DE-A-1937699 teaches a method and an apparatus for controlling a drive assembly for a web-shaped recording medium, which has been preprinted in pages in which a sensor is provided for scanning the recording medium. DE-A-2526190 as well as U.S. Pat. No. 4,994,975 and JPA-5902068 teach such methods. It is common to all these methods that they stipulate specific characteristic attributes of the mark, for instance with respect to the color of the mark or the contrast between the mark and its background surface, in order to be able to employ the respectively prescribed sensor. Therefore, the recording medium must be printed in a particular way.
De-A-19631747 teaches an optoelectronic sensor device, which works in conjunction with a light source, for detecting a marking that is known in advance on a web-shaped recording medium in electrographic printing or copying devices as well as a device for controlling the sensor device. It is provided there that the type of measurement method, or respectively, the selection of a required light source, is decided depending on a known mark that is provided on the recording medium.